The present invention relates to an ultrasonic Doppler blood flow meter apparatus for measuring blood flow in living beings by utilizing an ultrasonic Doppler method in the medical field for thereby generating blood flow images.
There is known a two-dimensional ultrasonic Doppler blood flow meter apparatus in whcih a blood flow distribution in a living being is determined by making use of an ultrasonic Doppler's effect and displayed in superposition on a two-dimensional monochromatic tomograph with the blood flow distribution being displayed is a color. This apparatus is also known as a color flow apparatus. The structure of the two-dimensional Doppler blood flow meter apparatus known heretofore is shown in FIG. 7.
Referring to FIG. 7, an ultrasonic transmitter 1 emits ultrasonic pulses through the medium of a probe 2 for irradiation of a portion of a living being. Echoes generated as the result of the irradiation with the ultrasonic pulses are converted into an electric signal through the probe 2, which signal is then amplified by a receiver 3 and undergoes phase shift detection by a phase shift detector 4 to be thereby converted to a Doppler shift signal. The Doppler shift signal outputted from the phase shift detector 4 is converted into digital data through an analogue-to-digital (A/D) converter 5, the digital data then being inputted to a wall filter 6 which is generally constituted by a FIR- or IIR-type filter of degree several and which serves to eliminate unwanted signal components of low frequencies (usually referred to as the clutter) which originate in intravital tissues. FIG. 6 shows, by way of example, a major portion of the wall filter constituted by an IIR-type filter of second degree or order. The wall filter 6 of this configuration is featured in that the characteristic equivalent to that of the FIR-type filter can be realized by using a smaller number of multipliers and adders when compared with the FIR-type filter and that the cut-off characteristic can flexibly be modified by changing the feedback coefficients K1 and K2. The Doppler shift signal from which the clutter signals are eliminated by the wall filter 6 is supplied to a blood flow rate arithmetic unit 7 which is adapted to determine arithmetically the blood flow rates. The output of the blood flow rate arithmetic unit 7 is then inputted to a digital scan converter (hereinafter referred to as the DSC in abbreviation) 9 together with a B-mode signal outputted from an envelope detector 8. In the DSC 9, the B-mode signal and the blood flow rate signal are mixed together, whereby a two-dimensional blood flow image is displayed on a screen of a monitor display unit 10.
In the ultrasonic Doppler blood flow meter apparatus known heretofore, it is desirable to impart steep characteristics to the wall filter 6 in order to provide highly accurate blood flow information. In that case, however, remarkably large transient responses tend to take place, making it difficult or impossible to obtain the blood information with high accuracy because the data contains the transient responses. Under the circumstances, when the blood flow rate is to be arithmetically determined with reasonable accuracy, much of the data will have to be discarded, resulting in that the amount of data supplied to the blood flow rate arithmetic unit will significantly be decreased, resulting in a problem that S/N ratio is degraded because the number of samplings decreases. On the other hand, when the number of data samplings is increased in order to compensate for the data lost, it is then required to increase the number of times the ultrasonic pulse is sent out and the echo is received in the same direction, which will however result in degradation in the frame rate.